1. Field of the Invention
This invention relates to a sealing arrangement for air heaters, and more particularly to an improvement to seals used on air heaters that cool the exhaust gasses and preheat the combustion air of organic fuel boilers. The present invention provides a more reliable seal that is less susceptible to wear.
2. Description of The Related Art
Air heaters are commonly used on boilers and furnaces to cool the resulting exhaust gasses and to preheat intake air by transferring energy from the exhaust gasses to the intake air. The raised temperature of the intake air supplied to the boiler allows combustion to be more complete and efficient. Since the heat contained in the exhaust gasses would otherwise be wasted, the use of this heat, to preheat the combustion intake, increases efficiency of the combustion process thus saving energy.
Before discussing the purpose and function of air heater seals it is important to understand the construction and function of a typical air heater. The air heater consists of a rotor in a housing. The rotor is of a cylindrical shape, several feet in diameter and several feet in height. Vertical dividing plates divide the rotor into equal sized pie-shaped sections, these plates extend along the full radius, and height, of the rotor. Within each section of the rotor is a heat exchanging element, consisting of an arrangement of materials of high thermal conductivity, usually metal.
The housing surrounding the rotor includes sector plates which close portions of the top and bottom of the housing and divide it into halves. One half of the housing has an inlet for cool air at the bottom and an outlet for heated air at the top. The other half of the housing has an outlet for cooled flue gas at the bottom and an inlet for hot exhaust gasses at the top. In operation, the rotor is rotated about its central axis causing the heat exchanging elements to be placed, alternately, into the flow path of the flue gasses and the flow path of the air. The arrangement resembles a revolving door with entrances and exits at the top and bottom instead of the sides.
When the heat exchanging elements are placed into the flow path of the hot exhaust gasses from the boiler, the element absorbs some of the energy of the gasses, cooling the gasses and heating the element. When rotation of the rotor then places this hot element into the path of the boiler inlet air supply the air is heated and the element is cooled. The resulting heated air may also be used for drying pulverized fuel or for other applications. This cycle is repeated for each heat exchanging element while the air heater is in operation. The result is that energy normally lost in heat of the exhaust gasses is recycled.
In order for the air heater cycle to be efficient, each section containing a heat exchanging element must be isolated from all other sections in order to prohibit gas leakage around the cool and hot elements. Further losses, due to leakage from the pressurized air side to the gas side of the air heater, which is typically in a state of relative vacuum, must be limited. For this purpose several styles of flexible seals have been developed that allow the rotor to rotate while somewhat prohibiting undesirable energy exchange, due to convection, between the elements.
The most popular materials used for seals are single or multiple metal leaf elements. These seals are popular because of their flexible characteristics. Since there are large temperature gradients within an air heater, deformation of the rotor and housing will occur. Flexible seals can bend and compensate somewhat for the changing gap between the rotor and the top, bottom and sides of the housing. Deformation also occurs as a result of poor construction tolerances or sagging due to gravity. A known air heater and sealing arrangement are illustrated in FIG. 1. The rotor 50 revolves around rotor post 17 in a direction indicated at D. Rotor 50 consists of rotor shell 8 and several vertical section dividing plates 35 extending radially outward from rotor post 17 to rotor shell 8 thus dividing rotor 50 into several pie shaped sections. A heat exchanging element (not shown) resides in each pie shaped section.
Rotor 50 is surrounded by housing 1 (shown in section) at the perimeter of rotor 50. Attached to housing 1 is connecting flange 10, facilitating connection to the boiler system ductwork (not shown). To prohibit leakage of air between each pie shaped section flexible radial seals 52 are located along the top and bottom edges of section dividing plates 35. Axial seals 54 are attached to the outer surface of rotor shell 8 prohibiting the flow of air between rotor shell 8 and housing 1. Finally, a circular seal 60 (shown in section) made of a thin flexible metal is attached to the top and bottom edges of rotor shell 8 so as to prohibit vertical air flow between rotor shell 8 and housing 1. Circular seal 60 extends around the entire circumference of rotor 50.
It is well known to construct these seals of thin, flexible metal or another solid, temperature resistant material.
The seals must be adjusted to seal when the gaps are the largest. This means that when the gaps are small the seals will be severely bent and forced into a high contact pressure with the rotor or housing. For this reason seals known in the prior art wear relatively quickly, causing replacement, air to gas leakage as great as 20 percent, downtime and great expense.
This problem has been addressed in the design of some more recent air heaters that have incorporated a complex electrical control system and hydraulics to bend the housing into conformation with the rotor as it distorts due to temperature gradients. This type of a system is expensive, and requires maintenance of a complex electro/hydraulic system as well as maintenance of the existing seals.
In addition, it has been known to utilize metal brushes as a seal between moving parts such as is disclosed in U.S. Pat. No. 4,398,508 to Moon et al. The Moon patent is for a seal between a fan and its shroud. This type of seal is designed for a small gap of fixed distance between machine surfaces.